1. Field of the Invention (Technical Field)
The present invention relates to an electrochemical sensing apparatus and method for continuous monitoring of metals and organic compounds, by means of continuous circulation of reagent and repeated introduction of fresh reagent.
2. Background Art
Detection and monitoring of metals and organic compounds is normally done by having an operator collect on-site field samples and then taking the samples back to a laboratory.
Attempts have been made to provide both remote field sampling and analysis, on-site. See U.S. Pat. No. 5,120,421, entitled "Electrochemical Sensor Detector System and Method," to Glass et al., and U.S. Pat. No. 5,296,125, entitled "Electrochemical Sensor Detector System and Method" to Glass et al. However, an operator is still required to be present on-site to collect the sample, and the device does not communicate directly with the laboratory.
Contamination of hazardous sites and groundwater with toxic heavy metals (e.g., mercury (Hg), lead (Pb), uranium (U), arsenic (As), chromium (Cr), aluminum (Al)) represents a major national problem. Site monitoring and surveillance programs are required for a closer control of metal pollutants. The traditional use of atomic-spectroscopy central-laboratory measurements of heavy metals is too expensive and time consuming. Also, samples often change composition during their collection, transport and delay before analysis, ultimately producing unreliable results. Innovative field deployable methods are highly desired for the task of site characterization and remediation, as they minimize the huge labor analytical costs, and provide timely data for real-time emergencies and decision making. Chemical sensors are particularly attractive for providing real-time, remote monitoring of priority pollutants. While fiber-optic probes have been suggested for monitoring organic contaminants, no chemical sensor technology has demonstrated capability for remote monitoring of trace metals. (See W. Chudyk, et al., J. Anal. Chem. 1985, 57, 1237.) Clearly, a cost effective metalsensor technology, capable of monitoring the metal both in time and location, is needed to support the characterization and remediation of hazardous waste sites. (See G. Batiaans et al., (Eds.), "Chemical Sensors: Technology Development Planning," U.S. Department of Commerce, Springfield, 1993.)
In the present invention, there is provided a sensor for in-situ monitoring of trace metals and organic pollutants. The sensor of the invention is designed both for remote, on-site, use or for use in laboratory applications. The compact instrumentation and low power needs of electrochemical techniques satisfy many of the requirements for on-site metal analysis. Particularly attractive for in-situ monitoring of metal contaminants is the remarkably sensitive technique of stripping analysis. (See J. Wang, "Stripping Analysis: Principles, Instrumentation, and Applications," VCH Publishers, Deerfield Beach, Fla. 1985). The extremely low (subanomolar) detection limits of stripping analysis are attributed to its "built-in" pre-concentration step, during which the target metals are deposited onto the working electrode. However, since the performance of stripping analysis as previously achieved, depends on the electrolytic plating of target metals onto the working electrodes of a sensor, conventional stripping analysis alone is not satisfactory for many environmentally-important metals that cannot be readily electroplated. Additionally, conventional stripping measurements, have suffered from interferences by surfactants (surface-active materials) commonly present in environmental samples that have passivated the electro-surface.